This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        BW bandwidth        CDM code division multiplexing        C-MIMO cooperative MIMO        CoMP coordinated multi-point transmission/reception        CRS common reference signal        CSI channel state information        DCI downlink control information        DL downlink (eNB towards UE)        DM-RS demodulation RS        DRS dedicated reference signal        eNB E-UTRAN Node B (evolved Node B)        EPC evolved packet core        E-UTRA evolved UTRA        E-UTRAN evolved UTRAN (LTE)        HARQ hybrid automatic repeat request        ID identity        JP joint transmission/processing        LTE long term evolution of UTRAN (E-UTRAN)        LTE-A long term evolution advanced        MAC medium access control (layer 2, L2)        MIMO multiple input multiple output        MM/MME mobility management/mobility management entity        MU multi user        MU-MIMO multi user MIMO        Node B base station        O&M operations and maintenance        OFDM orthogonal frequency division multiplexing        OFDMA orthogonal frequency division multiple access        PCI physical cell ID        PDCP packet data convergence protocol        PHY physical (layer 1, L1)        PRB physical resource block        QPSK quadrature phase shift keying        RE resource element        Rel release        RLC radio link control        RRC radio resource control        RRM radio resource management        RS reference signal        SC-FDMA single carrier, frequency division multiple access        SDM space division multiplexing        S-GW serving gateway        SI system information        SU single user        SU-MIM0 single user MIMO        TP transmission point        TPMI transmitted precoding matrix indicator        TTI transmission time interval        UE user equipment, such as a mobile station or mobile terminal        UL uplink (UE towards eNB)        UMTS universal mobile telecommunications system        URS UE-specific reference signal        UTRA UMTS terrestrial radio access        UTRAN universal terrestrial radio access network        WID work item description        
A communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) has been specified within 3GPP. The DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V9.1.0 (2009-Sep.), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 9)”, incorporated by reference herein in its entirety.
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE (not shown). The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (SGW) by means of a S1 interface. The S1 interface supports a many-to-many relationship between MMEs/S-GW and eNBs.
The eNB hosts the following functions:                functions for RRM: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
UE-specific reference signals (URS) (also known as dedicated reference signal (DRS) or demodulation RS (DM-RS) within the context of LTE-A) were agreed to be used as the demodulation reference signal in the downlink of Rel-10 and Rel-9. These reference signals are present in the transmitted physical resource blocks (PRBs) and the transmitted spatial layers. They undergo the same precoding operations as the corresponding data channel. Benefits of URSs include non-constrained precoding, no need for transmit precoding matrix index (TPMI) signaling in the downlink and reduced overhead compared to non-preceded common reference signals (CRS). See further, 3GPP TR 36.814, v0.4.1, “Further Advancements for E-UTRA Physical Layer Aspects”, February 2009 (attached as Exhibit A and incorporated by referenced in its entirety) and 3GPP WID RP-090359, “Enhanced DL transmission for LTE”, March 2009 (attached as Exhibit B and incorporated by referenced in its entirety).
Coordinated multi-point transmission/reception (CoMP) is considered a promising technique in LTE-A to achieve high cell-edge and cell average throughput gains. DRS mapping and initialization is to be used in LTE Rel-9 (dual layer beamforming) and in LTE-A (higher order DL SU-MIMO, MU-MIMO and CoMP) as well as in LTE-A Rel-10. See further, 3GPP TR 36.814 (Exhibit A); 3GPP R1-093890, “Considerations on Initialization and Mapping of DM-RS Sequence”, Nokia Siemens Networks, Nokia, October 2009 (attached as Exhibit C and incorporated by referenced in its entirety); 3GPP CR 0141R1, R1-095131, “CR 36.211 Introduction of enhanced dual layer transmission”, November 2009 (attached as Exhibit D and incorporated by referenced in its entirety) and 3GPP R1-093697, “Proposed Way forward on Rel-9 Dual-layer beamforming for TDD and FDD” August 2009 (attached as Exhibit E and incorporated by referenced in its entirety).
FIG. 2 illustrates an exemplary LTE-A Rel-9/10 URS scrambling sequence generation and mapping (for rank 1-2). The initialization and mapping of the URS scrambling sequences in LTE Rel-9/10 have the following properties: a re-initialization period of 1 subframe (1 ms) (sequence periodicity: one radio frame, 10 ms); a sequence for example, QPSK Gold initialized with Scrambling ID, Cell ID and subframe number; and a mapping, for example, the generated sequence assumes the maximum system BW in a subframe and maps to URS resource elements (REs) frequency-first (time-later) for the assumed system BW. The actual URS sequence may correspond to the used/allocated PRBs (pilots of multiple layers or SD multiplexed users can be separated by the use of an orthogonal code on top of the scrambling sequence). See further R1-093890 (Exhibit C) and R1-095131 (Exhibit D).
As shown, each RE in the resource grid represents a time-domain segment and frequency domain segment. As shown, one PRB encompasses twelve frequency domain segments of the system bandwidth and fourteen (or twelve for the extended cyclic prefix) time domain segments of the subframe (168 total REs or 144 total REs for the extended cyclic prefix). The PRB may be considered a PRB pair where the first seven (or six for the extended cyclic prefix) time domain segments represent even-numbered slots and the second seven (or six for the extended cyclic prefix) time domain segments represent odd-numbered slots.
The sequence of URS mapping progresses along the frequency domain for a first time domain segment then after reaching the maximum system bandwidth restarts in a first frequency domain segment at the next appropriate time domain segment.
FIG. 3 illustrates exemplary URS sequences for different BWs (the DC subcarrier is ignored for simplicity). In other relative frequency positions of transmission points with different bandwidths MU joint transmission/processing CoMP (JP CoMP) is not possible. As shown in FIG. 3, a given PRB pair (PRB with a given number/index) will use the same parts of the scrambling sequence regardless of the actual cell BW. This operation is sufficient for the case when 1) cooperating cells have the same BW and their center frequencies (e.g., DC subcarriers) are aligned or 2) cooperating cells have different BWs and their first PRBs (the lowest frequency PRBs) are frequency aligned. In other cases (e.g. when the BWs of cooperating cells are different and are center aligned), MU JP CoMP based on code division multiplexed (CDMed) DRS may not be possible due to scrambling sequences on overlapping PRBs of cooperating cells not being in-sync (e.g., not the same).
The problem of reference signal sequence variance due to different cell BWs was noticed for CRS in Rel-8. The Rel-8 CRS solution was to use the sequence re-initialization period of one OFDM symbol, generate the sequence for the max system BW and use the central part of the sequence in a given BW. Therefore, the CRS sequence in the central PRBs would not depend on the system BW.
Although the DRS sequence can also be centered so that its central elements are invariant to the system BW, the disadvantage of this solution is that JP CoMP with transmission points of different BWs would be only possible in case the center frequencies of transmission points are aligned.
See further 3GPP TS 36.211, v8.8.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, September 2009 (attached as Exhibit F and incorporated by referenced in its entirety) regarding the details of the Rel-8 CRS solution.
What is needed is a technique to sequence reference signals which can accommodate different cell BWs and does not require a specific frequency alignment.